How Big is a Small Whale?

The ongoing expansion of humanity has placed several animal species under threat of population decline or extinction, whether due to direct effects such as loss of wild spaces or more indirect ones such as climate change. (And a great many of these species are non-mammalian, of course, despite the focus of this blog). On the other hand, there have also been many conservation efforts that aim to restore, or at least preserve, species at risk. As I've previously described, for example, both black-footed ferrets and European bison went completely extinct in the wild during the 20th century, but are now back living in their original native habitats, if only in small numbers.

Not all such restoration efforts have been successful, and there can be many different reasons for this, not all of which are necessarily biological. Better then, of course, to try and prevent species from becoming endangered in the first place. For this reason, many species are protected even if they are not currently at risk. The more we can understand about these species, the better we will be able to keep them that way. 

One of the factors that can be useful here is an understanding of the species' demography; its population's age structure, how long individuals live, how many children they have, how long it takes them to grow up, and so on forth. This is more easily obtained for some species than others, with cetaceans being one of the groups that present particular challenges. Since cetaceans tend to be "protected" species due to the risks facing them, even if not all of them are officially "endangered" species, a better understanding of this can be helpful.

With whales and dolphins, you can't simply capture one in a baited trap, weigh and measure it, and then return it to the wild as soon as possible. They're also harder to observe than large land animals for which baited traps, at least, are equally impractical. Through the 20th century, by far the most common way of making these sorts of estimates for cetaceans was to examine those caught and killed by whalers. For example, this 1977 study estimated the growth rate and ages of a species of common, but rarely seen, whale based entirely on specimens caught by the Japanese whaling fleet. Most of those whales had been killed in 1957, twenty years before the study, so it's primarily an analysis of historical data that already existed... but where else would one get that information from?

Well, there are other places and, as whaling declined in the 21st century, those alternatives started to come to the fore. There are basically two options, which are to look at animals accidentally caught by other fishing trawlers, or those that are victims of natural stranding events. While the latter is becoming the more common sort of study in recent years, strandings are not that common, with the result that the majority of information we have is on the sort of species that were historically hunted, or at least tended to get caught in large fishing nets. 

The long-finned pilot whale (Globicephala melas) is a common species in many seas. Despite the name, it's technically a dolphin, albeit an exceptionally large one. Its closest relatives are also unusually large dolphins, but even so, the only "dolphin" that's noticeably larger is the killer whale, which belongs to a different subfamily.

Pilot whales are far from being an endangered species, and their worldwide population likely exceeds a million. But this is not to say that they face no threats, and they are at least partially protected by some international conventions, most notably appearing in Appendix II of the Convention of the International Trade of Endangered Species (CITES) which lists species that are thought to be at risk of becoming endangered unless their trade is tightly controlled. Threats to the species include disturbance from military and other forms of sonar, and bycatch by commercial fisheries.

The species is, in fact, still hunted, albeit in relatively small numbers by whalers from the Faroe Islands and, to a lesser, extent, Greenland. In this respect, it's notable that they are not protected by the usual rules enforced by the International Whaling Commission... because, so far as they are concerned, they are too small to count as whales and the rules don't apply to dolphins and porpoises. One positive is that the people of the Faroes (although not, so far as I know, Greenland) have been told by their government to stop eating pilot whales. That, however, is something of a double-edged sword since the reason given is that the whale meat contains high levels of poisonous mercury, which probably isn't much fun for the whales, either.

Even so, the fact that such whaling has been happening since at least the 16th century means that we do have a fair amount of information on the population demographics and size of the animals. But there's a complication: this only applies to one subspecies.

That's because long-finned pilot whales live in temperate waters of both hemispheres, with one population living in the North Atlantic and the western Mediterranean, and the other in a belt wrapping right around the southern parts of the globe. These populations represent different subspecies that likely have not been in contact for some time, and are separated by a wide belt of tropical waters where only the short-finned pilot whale is found. While this means that the southern subspecies (G. m. edwardii) is likely under less threat than the northern one - nobody seems to be hunting it - it also means that we know rather less about it.

Since there were some indications that the two subspecies might be significantly different demographically, a recent study examined records of over 1,500 long-finned pilot whales that were victims of (apparently natural) mass stranding events along the coasts of New Zealand between 1948 and 2017. 

In 405 of the individuals, enough had been preserved to determine their age at death. This is done by cutting thin slices of their teeth and counting the growth rings, rather as one would for a tree. (For anyone who does what I do for a living, and might care about the details, the teeth are ground down, decalcified in hydrochloric acid, cut at 25 µm on a freezing-sledge microtome, and stained with haematoxylin only). On this basis, the researchers determined that the oldest male in their sample was aged 31 years at the time of death, and the oldest female 38. 

This is rather lower than one might expect, since studies of whales caught off the Faroes and Japan have found some individuals much older than this, with one female having reached the age of 64. It's unclear whether this is a real effect, with the southern subspecies being shorter-lived than the northern one, however, because the northern samples don't come from stranded individuals and it might be that older animals are less likely to strand, or more likely to escape if they do. On the other hand, they do seem to reach sexual maturity more quickly, so a shorter lifespan isn't out of the question.

Which brings us to the rather tongue-in-cheek question asked in the title of this post: just how big is a small whale? The largest whale in the sample measured 622 cm (20' 5") in length, rather shorter than the record of 670 cm (22 feet) recorded for the northern subspecies, and the average lengths for adults were also smaller. Given the size of the sample, it's highly likely that this is a genuine finding, and it may have relevance for estimating whether or not a living individual has reached adulthood or is still growing. Or, for that matter, if their growth is being restricted by a sudden unavailability of food.

Combining these two pieces of information it was also possible to deduce how the whales grow throughout their lives, comparing age with size. The smallest whale in the sample, a newborn calf, measured just 160 cm (5 ' 3") and again, other newborns were noticeably smaller than those recorded from the northern subspecies. From there, it would seem that they grow fairly rapidly until they reach sexual maturity, at which point the growth rate of females drops off dramatically, although it never seems to stop entirely, even when they're quite elderly. 

Males, however, while initially only slightly larger than the females, put on a sudden growth spurt at around eleven years. That lasts for about a year, but even after that, males grow more rapidly than females of the same age do, only really slowing down again once they reach 25 or 30. This suggests that female pilot whales find large males sexy, since eleven years is about the time that the males are reaching sexual maturity. Females reach maturity much earlier, around 7 years old and it's likely that, from this point on, they put their energy into pregnancy and raising the resulting young, rather than simply growing for the sake of it. For the males, if larger size means more mating opportunities and more offspring, it certainly isn't a waste.

Of course, we already knew that male pilot whales are larger than females, sometimes significantly so. In this sample, for instance, that 622 cm adult was, of course, a male, while the largest female measured 500 cm (16' 5") long. But this sudden growth spurt is a new finding, albeit one also seen in, for example, bottlenose dolphins. It might, perhaps, reflect a difference between the subspecies.

It's not just the overall body that grows, either. Males have larger fins and tail flukes than females of the same body size - and this is more extreme in the southern than the northern subspecies. Unlike other sexually dimorphic animals, such as seals or deer, male pilot whales don't fight one another, so these differing bodily proportions aren't linked to any direct physical competition. Possibly, larger fins and flukes may give them greater stability in the water, or enable them to swim faster. Or, because zoologists always like to consider this sort of possibility, females just think big fins are sexy. 

Or all of these are true, since they're not exclusive.

[Photo by Barney Moss, from Wikimedia Commons.]